The present invention relates to continuously variable belt-drive transmission for a motor vehicle.
A known continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid-operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions.
FIG. 3 shows the conventional continuously variable belt-driven transmission for a motor vehicle.
The belt-drive transmission has a main shaft 4 and an output shaft 9 provided in parallel with the main shaft 4. A drive pulley 1 and a driven pulley 6 are mounted on shafts 4 and 9 respectively. A fixed conical disc 2 of the drive pulley 1 is integral with main shaft 4 and an axially movable conical disc 3 is axially slidably mounted on the main shaft 4. The movable conical disc 3 also slides in a cylinder 16 formed on the main shaft 4 to provide a servo device. A conical face 2a of the fixed conical disc 2 confronts a conical face 3a of the movable conical disc 3 thereby forming a groove 5 therebetween.
A fixed conical disc 7 of the driven pulley 6 is formed on the output shaft 9 opposite a movable conical disc 8. The movable conical disc 8 is slidably engaged in a cylinder 17 on the output shaft 9 to form a servo device. Conical faces 7a and 8a of the respective discs 7 and 8 form a groove 10. A belt 11 engages the drive pulley 1 and the driven pulley 6.
The belt 11 comprises a plurality of metal elements 12 adjacently arranged in the longitudinal direction of the belt and each having a pillar portion 14 at the center and horizontal slits at both sides thereof, and a seamless laminated endless metal carrier 13 inserted in the slits.
Each element 12 has a dimple 12a on its one side and a cavity 12b on the other side. The dimple 12a of one element 2 engages with the cavity 12b of the adjacent element and all elements are arranged side by side. The belt is thus assembled.
When the movable conical discs 3 and 8 are axially moved along the shafts 4 and 9 for changing the transmission ratio, the center lines of the grooves 5 and 10 move relative each other. Such a misalignment (hereinafter, called offset) causes the edge of carrier 13 of the belt 11 to rub against the pillar portions 14 of the elements 12 or the conical faces 2a, 3a and 7a, 8a of the pulleys. Additionally, running of the belt 11 becomes unstable when the entering pulleys. As a result, the belt easily wears out and the conical surfaces become rough.
Referring to FIGS. 7 and 8, the length L of the belt is EQU L=(.pi.+2.phi.)R.sub.P +2D cos .phi.+(.pi.-2.phi.)R.sub.S ( 1)
where R.sub.P and R.sub.S are the radii of the drive and driven pulleys respectively.
If the pulley ratio is 1, namely R.sub.P =R.sub.S =R, the above equation (1) can be written as EQU L=2D+2R.pi. (2)
From FIG. 7, EQU R.sub.P =R.sub.S +D sin .phi. (3)
When equations (2), (3) are substituted for Eq. (1), ##EQU1##
On the other hand, supposing the offset is zero when the pulley ratio is 1 (R.sub.p =R.sub.S =R), the offset .DELTA.X can be represened as (FIG. 8) ##EQU2##
The amount of the offset .DELTA.X between the center lines of the pulleys 1 and 6 can be calculated from the following formula. EQU .DELTA.X=2D/.pi.{(1-cos .phi.-.phi. sin .phi.)}tan .beta. (7)
where D is a distance between the centers of the drive and driven pulleys, .phi. is an angle between the linear portion of the belt and the line connecting the centers of the pulleys and .beta. is the angle of inclination of the conical face.
When D=140 mm, .beta.=11.degree. and R=49.2 mm, in which R is the effective radii of the drive pulley 1 and driven pulley 6 when the pulley ratio i is 1, and assuming that the pulley ratio i varies in the range between 2.504 and 0.498, the amount of offset .DELTA.X varies in the range between 0 and 0.734 mm. In a conventional design, the initial set value of the pulley arrangement should employ the medium of the range 0.734 mm of the offset amount. Both pulleys are disposed so that the amount of the offset varies between .+-.0.367 in the entire pulley ratio range (2.504-0.498), as shown in FIG. 4.
Therefore, the above mentioned formula (7) is rewritten as follows: EQU .DELTA.X=2D/.pi.(1-cos .phi.-.phi. sin .phi.) tan .beta.-0.367 (8)
Experiments concerning sensitivity of the belt in relation to the offset at each pulley ratio have shown that when the offset .DELTA.X exceeds a permissible value, .DELTA.Xp, the belt is distorted and the running thereof becomes unstable. This occurs because when the belt makes a curve along the circumference of a pulley, the elements tend to rotate about the dimples or about the pillar portions. Ranges of permissible offset values .DELTA.Xp for typical pulley ratios are shown by arrows in FIG. 5 wherein the vertical line and the horizontal line indicate the permissible offset amount .DELTA.X.sub.P and pulley ratio i, respectively. As can be seen from the graph of FIG. 5, the range of the permissible offset value .DELTA.Xp is the narrowest when the running belt is at the maximum speed, namely the pulley ratio is at a lower value, which is hereafter called pulley ratio Top through the specification. FIG. 6 can be obtained from FIGS. 4 and 5, showing the relationship between the variation of the offset and the permissible offset value.
In these figures, Low of the pulley ratio corresponds 2.504, Med 1.0, Top 0.7, OD (Over Drive) 0.498, respectively.
It is apparent from FIG. 6 that the offset .DELTA.X at the pulley ratio Top exceeds the range of the permissible value .DELTA.Xp. Accordingly, at the pulley ratio Top, that is when the running speed of the belt is at maximum (Vmax), the elements of the belt become unstable, greatly affecting the durability of the belt. U.S. Pat. No. 4,596,536 discloses a method for resolving the above described problems.